The present invention relates to a method for producing an article having a microstructured surface, and the resultant article. The surface of the article thus forms an interface between the article and the adjacent medium, which if of differing indices of refraction, results in enhanced light transmission and decreased reflectance without producing significant diffuse scattering.
Various types of coatings to reduce reflectivity and improve the transparency of articles such as lenses and windows, and to improve the efficiency of solar cells and solar absorption panels are well known. Perhaps the best known are the single, or preferably multiple, layer interference coatings used on optical lenses, filters and as antireflecting films used on windows. While such coatings are desirable in that they are durable and are known to provide an extremely low reflectivity at specific wavelengths, they exhibit a number of limitations. For example, the optical characteristics of such single layer films are highly sensitive to the wavelength, such that multiple layer coatings must be employed. However, if such multiple layer coatings are used, a significant sensitivity to the direction of incident light results. Interference coatings providing antireflecting characteristics which are simultaneously independent of the incident wavelength and in which the antireflection is substantially uniform over a wide range of incident angles, are, therefore, not known. Furthermore, such interferrometric films are relatively expensive to produce, requiring careful control of the thickness of the coating as well as multiple coating operations.
In addition to such articles in which the reflectance therefrom is reduced via a coating having optical interference characteristics, it is also known to provide articles in which the reflectance is reduced by providing a microstructured surface over which the effective index of refraction varies continuously from the substrate to the ambient environment. See, for example, U.S. Pat. No. 2,432,484 (Moulton) and the above referenced patent to Maffitt et al, which patent is assigned to the present assignee. It is believed that the highly sensitive vision of nocturnal insects, such as moths, is at least partly due to the low reflectivity from the surface of the eyes due to the presence of such a microstructure on the surface of the eye. G. C. Bernhard et al, Acta Physiologica Scand., Vol. 63 243, pp. 1-75 (1965).
Another example of a method of producing an antireflective surface utilizing a regular array of microprotuberances is disclosed in U.S. Pat. No. 4,013,465 (Clapham).
Solar collectors utilizing porous coatings to increase the absorptivity and to minimize the radiation loss due to reverse reflected radiation (visible or IR) are also known. It is also known to utilize micropores, grooves or other "textural" effects in such devices to effect an increase in absorptance. J. Vac. Sci. Tech., Vol. 12, No. 1, Jan/Feb (1975). For example, U.S. Pat. No. 3,490,982 (Sauveniere et al) discloses a method of treating a glass surface to provide a microstructured surface exhibiting reduced reflectivity. Commercial acceptance of some of the coatings, surface treatments and the like disclosed in the above cited references have not proven commercially acceptable, possibly due to the instability of the surfaces, cost or inability to provide uniform surfaces over extended areas.
Articles having a microstructured surface are also disclosed in U.S. Pat. No. 4,190,321, (Dorer & Mikelsons) which patent issued Feb. 26, 1980, and is assigned to the same assignee as the present invention. That patent discloses the treatment of an aluminum surface to form thereon an aluminum hydrate, or boehmite composition having a plurality of randomly positioned leaflets which give the treated surface an antireflecting characteristic. In a somewhat similar manner, U.S. Pat. Nos. 3,871,881, 3,975,197 and 4,054,467 disclose prior inventions of Mikelsons in which aluminum surfaces are treated to provide microstructured boehmite surfaces by which other coatings, applied to the aluminum prior to treatment, become tenaciously bonded to the aluminum. Also, U.S. Pat. No. 3,664,888 (Oga et al) depicts an electrochemical process for treating aluminum or aluminum alloy surfaces which etches the surface, leaving minute irregularities and pinhole cavities which are said to provide mechanical anchorage for subsequently applied resin coatings.